Soil Sampling And Variability – What Does This Mean For Your Nutrient Decision?

GRDC - Yvette Oliver, Phil Ward, Karen Treble

Type: Research Paper
Knowledge level: Advanced

Farm Table says:

The authors of this paper note in the conclusion: "The level of nutrient variation coupled with the number of sample cores taken will affect how closely soil test values represent the actual nutrient level of a soil. By using an approach that calculates the economic impact of under- or over-fertilising, the optimal soil sampling strategy can be determined." Please access the full paper via the link below if this research interests you.

The take home messages from this GRDC funded research are below. Please access the full paper via the link below for methodology, references, acknowledgements and discussion.

Take key messages from the paper include:

  • Nutrients such as Nitrogen (N), Phosphorous (O), Potassium (K) and Sulphur (S) can vary within even a small transect on the same soil type. This variability is measured using the coefficient of variation (CV%). In soil sampling, the CV% can range from 10-80%.
  • The more samples bulked before lab analysis, the lower the CV% and the more accurate the soil test value.
  • At low nutrient variability (CV of 20% or less) the optimal number of cores that should be taken are around 8-10 while at higher variability (CV of 50% or higher) the optimal number required rises to 20-30.
  • The degree to which nutrient variation and sampling strategy affect fertiliser decisions will depend on the risk profile of the farmer or consultant.
  • Using the tool ‘NPK’ decide we demonstrate the economic impact of applying an average fertiliser rate to a paddock when the soil test values are either higher or lower than the average. We then use this economic impact to determine an optimal sampling strategy for the paddock.

2020 - Autralia - GRDC - Yvette Oliver, Phil Ward, Karen Treble
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